This invention relates to a signal amplifier circuit and more particularly to a signal amplifier circuit utilizing a field effect transistor having current unsaturated triode vacuum tube characteristics.
Recently, a field effect transistor having current unsaturated characteristics like those of a triode vacuum tube disclosed in U.S. Pat. No. 3,828,230 issued to Nishizawa et al. on Aug. 6, 1974 (the corresponding Japanese patent application was laid open on Mar. 30, 1973) has attracted attention as an active amplifying element for an audio signal amplifier circuit.
A field effect transistor and bipolar transistor hitherto used as active amplifying elements have current saturated characteristics like a pentode vacuum tube. The aforesaid new field effect transistor excells over these old amplifying elements in respect of output power, linearity and frequency characteristics.
The fact that an amplifying element has triode vacuum tube characteristics, means that the operating current of the amplifying element varies not only with an input signal, but also with variation in direct-current supply voltage. Where, therefore, it is attempted to design a direct-coupled signal amplifier circuit using a current unsaturated type amplifying element, then same kind of the conventional direct-coupled amplifier circuit using a current saturated type amplifying element can not be applied in the original configuration.
A true-complementary or quasi-complementary pushpull circuit has been widely used as an output stage for an audio signal amplifier. As a driver circuit to drive the complementary pushpull circuit is known a bootstrap driver circuit. Where a new current unsaturated type field effect transistor is used as an active amplifying element of the bootstrap driver circuit, then a gain in the driver stage prominently falls due to an output signal from the pushpull circuit being superposed on direct-current operating voltage supplied to the driver stage, failing to drive fully the pushpull circuit.